Image display



Dec. 20, 1960 E. E. LOEBNER IMAGE DISPLAY 2 Sheets-Sheet 1 Filed NOV.27, 1956 8 E Mm u. u. 3 n 6 n u.' G \2| F 8 2 2 2 .v 6 0 3 2 f HT! 2nn\` AK l3 f o O "v" /18 2 I I l/ I I O mln Il n 3 X m HG I F 3 BY 770mm077m@ ATTORNEY 2 Sheets-Shea?l 2 E. E. LOEBNER IMAGE DISPLAY Tm; MP4@ lO NMFZDOU Dec. 20, 1960 Filed NOV. 27, 1956 INVENTOR EGON E. LOEBNER BYifo/wm (9 ATTORNEY United States Patent O IMAGE DISPLAY Egon E. Loebner,Princeton, NJ., assigner, by mesne assignments, to Sylvania ElectricProducts Inc., Wilmington, Del., a corporation of Delaware Filed Nov.27, 1956, Ser. No. 624,633

4 Claims. (Cl. 315-169) This invention relates to image display devicesand more particularly to an image display device of the llat typecomprising a plurality of structurally similar discrete image elements.

Prior art teaches the concept of energizing electroluminescent phosphorsthrough the use of a cross conductor mesh arrangement whereby theelectroluminescent material in a given image area may be energized by avideo voltage so as to produce video modulated light. Though suchstructures show commercial promise, it is apparent that improvement ofthe light output or efliciency of the electroluminescent layer isnecessary in such displays. When a screen of electroluminescent materialis combined with a sweep system which contemplates very brief intervalsof video excitation, the resulting light output values, as yet, are notcomparable to those produced by present day cathode ray tube screens.

This invention contemplates increasing available average light outputfrom an electroluminescent image screen by providing a time period oflight emission greater than the time period of video signal excitationof any given elemental image area. This is accomplished -by a conceptmaking it possible to take advantage of the relatively long time periodavailable between successive energizations of any given elemental imagearea.

Thus, it is an object of this invention to provide available increasedlight output from an electroluminescent display.

It is a further object of this invention to utilize the light outputcharacteristics of electroluminescent materials to increase availabletotal light output of an image display.

It is also an object of lthis invention to provide peak available lightoutput from each elemental area of an electroluminescent display whichis monotonically related to a characteristic of lthe exciting signalvoltage.

Briefly, in one aspect of the invention I provide a ilat image displayelement comprising a plurality of layers having one layer for producinglight related to the amplitude and time duration of a signal voltage anda second layer excited vby the light output from said lirst layer forproducing light having an available peak intensity monotonically relatedto the light output of said lirstlayer for a time duration, longer thanthe original signal voltage duration.

For a better understanding of the invention, together with other andfurther objects and capabilities thereof, reference is made to thefollowing description and appended claims in connection with:

Fig. l showing the viewing layer of the image display element of anembodiment; and

Fig. 2 showing the video signal excited layer of the image element of anembodiment; and

Fig. 3 showing the elements of Fig. 1 and Fig. 2 in combination;

2,965,802 Patented Dec. 20, 1960 ICC Fig. 4 showing a plurality of imageelements in specilic embodiments; and

Fig. 5 is a scanning system useful with the image display device of thepreceding figures.

In Fig. 1 there is shown a cross section of a display I image elementcomprising what might be termed an elemental sandwich of layers having atransparent conductive layer 11, an electroluminescent layer 13, asemilight-absorbing layer or filter 15, which may be conductive, aphotoconductive layer `16 and a second transparent conductive coating18. Alternating current exciting source 20, which may be in themegacycle range, is shown connected between the transparent conductivelayers |11 and -18.

When the surface :18 of the element shown in Fig. 1 is exposed to asource of radiant energy selected preferab-ly from the visible or nearvisible portion of the light spectrum, there is a decrease in theimpedance of photo-conductive layer 16. Thus, there is an increase inthe voltage impressed across electroluminescent layer 13. If the lightemitted from electroluminescent layer 13 is in the spectral range as toalso decrease the resistance of photoconductive layer 16, it can be seenthat the complete unit operates somewhat similar to a unistable triggerdevice in that light feeding back through the filter layer 15 tends tofurther decrease the resistance yof photoconductive layer '16 and thusfurther increase =the amplitude of the excitation voltage applied toelectroluminescent layer 13. This regenerative action continues evenafter removal of the radiant energy excitation of surface 18 and quicklydrives the unit further toward a given light output at a rate and towarda peak output primarily governed by the intensity of the originalexciting light passing through surface 18 to photoconductive layer 16and partly controlled by the intensity of the light fed back throughlilter layer 15. By selecting the 'light absorbing characteristics offilter 1S so as to limit the amount of light fed back fromelectroluminescent layer A13, it is possible to limit the time period ofthe unstable condition to a period somewhat less than the maximum periodbetween successive energizations of an image element in known televisionsystems. Thus, after the external source of radiant energy is removedthe intensity of the light output from layer 13 continues to increase toa peak value related to the peak intensity of the externally appliedradiant energy and then starts to decrease. At this point the resistanceof the photoconductive layer 16 starts to increase, thereby de creasingthe voltage `across the electroluminescent layer 13. This in turndecreases the amount of light fed back to the photoconductive layer |16and as a result, the element returns to the stable condition where nolight is lbeing emitted from layer 13.

The decay rate, after external radiant energy excitation is removed,depends not only upon the lter characteristics of light absorbing layer=15, but also upon the photoconductive characteristics of layer 16. Forexample, by selecting a photoconductive material for layer 16 whichresponds relatively rapidly to radiation in the spectral band of theexternal radiating source and somewhat slower to radiation in thespectral band of the light emitted by the electroluminescent layer 13,it is possible to make the resistance of the photoconductive materialdecrease rapidly when excited by light from the external source and thenincrease relatively slowly after the external source is extinguished.

Thus, it can be seen that the element shown in Fig. 1 is stable in theextinguished condition until temporarily excited by Vexternal radiantenergy whereupon it assumes` long as the maximum value of the externallight source is not such as to allow saturation of photoconductive layer16, an increase in the amount of instantaneous light from the externalsource will accelerate the rate at which the resistance of thephotoconductive layer decreases; however, the rate at which theresistance of the photoconductive layer returns to the extinguishedcondition value remains essentially independent of variations inexciting source amplitude and duration.

As can be seen, the unit of Fig. 1 provides means for producing lighthaving a peak intensity monotonically related to the peak intensity ofan exciting light source.

In Fig. 2 I have shown an electrolumineseent exciting element comprisinga transparent conductive layer 22, supported by transparent panel 30, anelectrolumineseent layer 24 and a second conductive layer 26 supportedby panel 2S. Layer 26 and panel 28 may or may not be transparent. Avideo voltage, not shown, is coupled between the two terminals 32 and 34which are connected to the two conductive layers 22 and 26,respectively, so as to establish an exciting iield acrosselectrolumineseent layer 24 which is proportional to the videoinformation received for the image element in question. Though notshown, additional pulse voltages may be necessary to bring theelectrolumineseent material up to the light incipient state as morefully explained in copending application Serial Number 306,909, tiledAugust 28, 1952, by Norman L, Harvey. Thus, light output from theelement portion shown in Fig. 2 is directly related to the videoinformation supplied to the image element in question, being energizedduring only those brief intervals when the video signal carriesinformation necessary to the image element.

Present day black and white television uses a sweep system wherein animage element is briefly energized each 1,@,0 of a second. When such asignal is applied to the electrolumineseent element in Fig. 2 the timeaverage light output of the element may be too low for direct viewingbut yet suicient to excite the structure of Fig. l.

In Fig. 3 I have shown the structures of Fig. l and Fig. 2 combined as acomplete image element, primarily using the same reference numerals asused in Fig. l and Fig. 2. Thus the complete element is seen to comprisethe support layer 28, conductive layer 26, electroluminescent layer 24,transparent conductive layer 22, transparent support layer 30,transparent conductive coating layers 18 and 11, and electrolumineseentlayer 13 separated from photoconductive layer 16 by lter layer 15 whichmay be conductive. If desired, a further transparent supporting layer 36may be used.

Operation of the complete element shown in Fig. 3 is based uponoperation of the portion shown in Fig. 1 and Fig. 2. The video signalapplied to terminals 32 and 34 excites electrolumineseent layer 24 toproduce a light output which is related to the amplitude of the videosignal. The light from layer 24 passes through transparent conductivecoating 22, transparent layer 30 and transparent conducting coating 18to impnge upon photoconductive layer 16. The resistance ofphotoconductive layer 16 thus decreases, increasing the voltage acrosselectroluminescent layer 13, supplied by exciting voltage source 20.Light is thereupon emitted by layer 13 through transparent layers 11 and36 in the viewing direction and also through ilter layer 15 in thefeed-back direction. As was brought out in connection with operation ofthe element portion shown in Fig. l, the light fed back through lter 15further decreases the resistance of photoconductive layer 16 which inturn allows a greater portion of the voltage supplied from excitingsource 20 to be applied across electrolumineseent layer 13. Thisregenerative action continues at a rate primarily governed by theintensity of the exciting light emanating from layer 24.

When the video voltage is removed from terminals 32 and 34 light is nolonger produced in the exciting layer 24, and the resistance of thephotoconductive layer 16 starts to increase, thereby decreasing thevoltage across electrolumineseent layer 13. As a result, light outputfrom layer 13 starts to decay at a rate governed in part by the filtercharacteristics of layer 15 and the sensitivity of photoconductive layer16 to the light fed back. Thus it can be seen that the unit of Fig. 3provides means for producing a light output having a peak intensityrelated to the peak amplitude of a video signal voltage. Also, it shouldbe clear that the light output of the viewing layer 13 is monotonicallyrelated to the light output of exciting layer 24.

In Fig. 4 there is shown a preferred embodiment of a portion of a specicimage display screen which has been constructed. The screen comprises astructure having elemental areas basically similar to the structure ofFig. 3 in that it is composed of layers including an electrolumineseentexciting layer 40 sandwiched between a conductive layer 42 and atransparent conductive layer 44. Conductive layers 42 and 44, as shownin Fig. 4 may each comprise a plurality of parallel conductors in theform of a grid wherein the conductors of one of the grids, e.g., grid42, are arrayed vertically and the conductors of the other grid, e.g.,grid 44, are arrayed horizontally in parallel planes. Thus, byenergizing any given horizontal-vertical conductor pair, it is possibleto establish a field across a discrete portion of layer 40 so as tocause this portion of the layer to luminesce. Considering this smallportion of layer 40 as an image element it can be seen that energizationof a large number of such horizontal-vertical conductor pairs in a givensweep sequence will provide an equally large number of image elementsand form a complete image display. For a more complete disclosure ofcross-grid structures reference is made to the copending applicationsSerial Number 306,909, filed August 28, 1952, by Norman L. Harvey, andSerial Number 306,800, tiled August 22, 1951, now abandoned, by WilliamK. Squires, both of which are assigned to the assignee of thisapplication.

In the structure of Fig. 4, layer `46 may be a transparent supportinglayer. Immediately adjacent supporting layer 46 there is provided atransparent conductive layer 48. For each horizontal-vertical conductorpair the photo-conductive layer comprises a cadmium sulphide or othersuitable crystal 50 supported in a light conducting tube S2, which maybe of Lucite, having a rough internal ysurface and a smooth externalsurface. It has been found advisable to coat the external surface oftubes 52 with a light opaque lm so as to make the tubes impervious tothe passage of light horizontally between tubes. In order to connect thephotoconductive cadmium sulphide crystals 50 to the conductive filterlayer 54, I have also found it useful to depend upon a minute dot ofsilver paint or some other conductive paint. Electroluminescent viewinglayer 56, transparent conductive layer 58 and the transparent supportinglayer 60 complete the structure. An appropriate alternating voltagesource, similar to source 2D in Fig. 3, may be connected between layers48 and 58, for excitation.

Operation of the elements in Fig. 4 is similar to the operation of theelement in Fig. 3. When the video voltage is applied across anyhorizontal-vertical conductor pair in layers 42 and 44, the interjacentportion of electroluminescent layer 40 luminesces to provide arelatively low intensity light which passes through layers 44, 46 and 48to impinge upon the associated cadmium photoconductive crystals 50 anddecreases its resistance. In view of the voltage impressed betweentransparent conductive layers `4S and 5S, it can be seen that there is ashift in voltage distribution and a larger portion of the voltage eginsto be impressed across the associated portion of the electrolumineseentviewing layer 56. Light fed-back from layer 56 as ltered by layer 54further decreases the resistance of the pertinent cadmium crystal inregenerative manner. Then as the video information is removed fromacross thel particular portion of Ilayer 40 in question, light outputfrom the associated image element in layer 5-6 starts its decay towardthe stable non-excited condition. As was the case in connection withFig. 3 the decay period is governed in part by the filtercharacteristics of layer 54 and the sensitivity of the photoconductivelayer 50 to the light fed back. lt is to be noted that the delay must berestricted to a time period which is less than the period existingbetween successive energizations of the image element in question.

Through most complete displays require far more image elements thanshown in Fig. 4, it should now be clear that structures built inaccordance with this invention have image elements with increased totaltime average light output relative to the image elements ofelectroluminescent screens which rely solely upon light output from abrief video excitation period. The advantages of my concept do notdepend upon the type of scanning utilized in that any scanning structureor system may be used which is found suitable for energizing theelectroluminescent exciting layer. Further, even though improvements aremade in methods of extracting light from known electroluminescentmaterial and even though more eficient electroluminescent material maybe discovered in the future, it i-s believed that my concept makes itpossible to obtain substantially maximum light output possible from anygiven material.

Though the concept is not to be limited to any single means or method ofscanning an electroluminescent exciting layer, there is shown, in Fig.5, one scanning system believed to be suitable. This particular scanningsystem is more exhaustedly disclosed and claimed in a copendingapplication filed by Donald 1C. Livingston and assigned to the assigneeof this application, now United States Patent 2,774,813, issued December18, 1956.

Referring to lFig. 5, which is a lschematic representation, it can beseen that there is provided a plurality of grid cross-over points 100which are fed from `a plurality of parallel separate vertical conductors102, 1014 and 106, forming the first grid array of theelectroluminescent exciting layer.

The second grid array of the electroluminescent exciting layer comprisesa plurality of parallel separate horizontal conductors 108, 1.10 and.1112. Each cross-over point 100 i-s coupled to a separate image elementportion of the electroluminescent exciting layer 114. Each of theelectroluminescent exciting layer image elements 1114 is coupled to avertical conductor through a rectifier 1'16. Each of the verticalconductors 1.102, 104 and 106 is connected through an associatednormally open gate circuit 120, 122 and I124 to a source of negativepotential, indicated as -V. Each of the vertical conductors is alsoconnected through an associated normally closed gate 126, 1,28 and '130to a source of positive potential, shown as +V, in series with anincoming video signal connected across terminals 132. Counter I134 has aseparate output for each vertical conductor associated gate pair, i.e.,gate pair 120-126, gate pair 122- 1.128 and gate pair 124- 130.

Horizontal conductors 108, 1110 and 1'12 are each coupled through anassociated normally open gate 136, 138 and 140 to a source of positivepotential shown as +A. Also, each of the horizontal conductors iscoupled through an associated normally closed gate 142, 144 and 146 to asource of negative potential shown as V. Counter circuit 148 supplies aseparate output for each horizontal conductor associated gate pair1136-142, 138--144 and 140-146 Both counters 134 and 148 are suppliedwith synchronizing pulses from terminal 150. Since counter circuits andgate circuits are so well known to the art, it is not believed necessaryeither to give a detailed description of the internal circuitry of eachblock or show the common terminals necessary for incoming pulsedevelopment and pulse transmittal from the counter-s to the associatedgate circuits.

Each electroluminescent exciting layer image element 1-14 is associatedwith a photoconductive image element 152, a filter element 154 and anelectroluminescent final display element 156. 'Ihe exciting voltage forthe elements 154-156 are supplied from a source :160.

The potential of the source shown as +A is chosen so as to have a valuerelative to an arbitrary reference level, which exceeds the sum of thepotentials from source +V and the maximum potential of the video signalimpressed across terminals 132. The potential -V assists in back biasingrectifiers 116 to prevent improper conduction.

Operation of the circuit may be understood by considering a video signalwhich includes a synchronizing pulse for each line of the videoinformation. The video signal is applied across terminals 132 and thestripped synchronizing pulses, separated by a circuit not shown, areap'- plied at terminal 150. For each synchronizing pulse impressed onthe input of counter 32, the output provides a series of pulses or pulsetrain having one pulse for each vertical conductor in the complete imagedisplay. The first output pulse of `counter 134 simultaneously opensgate 126 and closes gate 120. At the same time counter 148 is triggeredinto supplying a pulse to close gate 136 and open gate 142. The counter148 differs from counter 134 in that counter 148 supplies one pulse foreach incoming sync pulse with the first output pulse appearing on lead162 and the second output p-ulse on lead 164 and the third output pulseappearing on lead 166 and so on depending upon the number of horizontalconductors uti- `lized in the complete display. Thus the first syncpulse, in opening gates 126 and 142 and in closing gates 120 and 136,allows the incoming video signal to be impressed on the firstelectroluminescent image element 114 at the upper left hand corner ofthe display, as sho-wn. After the normal excitation period of the oneimage element, counter 134 impresses a pulse between gates 122 'and 128while terminating the pulse fed to gates and 126. When gate 128 isopened the incoming signal is then applied to the secondelectroluminescent video element 114 for the period of the pulsesupplied by counter 134. After a period of time equal to the normalimage element excitation period, counter 134 supplies a pulse openinggate 130 and closing gate 124 and terminates. the pulse between gates122 and 128. Gates 122 and 128 then return to their normal position,i.e., gate 122 is open and gate 128 is closed.

Considering that the structure shown in Fig. 5 should be expanded toinclude the number of image elements desired in each given display line7it can be seen that counter 134 supplies pulses which effectively allowsthe incoming video signal to sweep horizontally across line 108. Whenthe last image element in line 108 is extinguished, the next pulse atterminal trips counter 148 to terminate its output pulse on line 162 andsupply an output pulse to 164 and the sequence is repeated.

As each electro-luminescent image element 114 in the exciting layer isenergized by the incoming video signal, its associated photoconductiveelement 152 is excited in accordance with the intensity of light soproduced by the element 114. As explained in connection with Fig. 4, theresistance of the photoconductive element 152 therefore decreasesallowing the greater share of the voltage supplied from source to beimpressed across the electroluminescent element 156. Electroluminescentelement 156 starts to luminesce feeding back light through filter 154 inregenerative fashion. As a result electroluminescent image element 156in the viewing layer is duiven toward a high light output having a peakintensity related to the amplitude of the video signal exciting theassociated element 114. Though successive elements 114 are swept withvideo information as previously described, element 156 may be allowed toluminesce for a period substantially longer than the excitation periodof the associated element 114. The decay rate of any overall imageelement need be no faster than the period between successive scans of agiven element. As a result there is an increased total time averagelight output relative to the total time average light output of theexciting layer 114, which is limited by the brief video excitationsignal period.

While there has been shown and described what is at present consideredthe preferred embodiment of the invention, it will ybe apparent to thoseskilled in the art that various changes and modiiications may be madetherein without departing from the invention as dened by the appendedclaims.

Having thus described the invention, I claim:

1. An element in an image display comprising the combination of aplurality of closely adjacent layers having an electroluminescentexciting layer, a source of vdeo information voltage signals coupledtoexcite said electroluminescent exciting layer for producing lightoutput varying substantially in accordance with the video information ofsaid voltage signals, a layer of light sensitive material havingimpedance characteristics varying in accordance with variations inintensity of light impinged thereon, said light sensitive layer beinglight coupled to said electroluminescent exciting layer, anelectroluminescent viewing layer electrically and light coupled to saidlight sensitive layer, an-:l a voltage source coupled across saidelectroluminescent viewing `layer `and said light sensitive layer.

2. An element in an image display comprising the combination of aplurality of closely adjacent layers having an electrolumincscentexciting layer, a source of video information voltage signals coupled toexcite said electroluminescent exciting layer for producing light outputvarying substantially in accordance with the video information of saidvoltage signals, a layer of light sensitive material having impedancecharacteristics varying in accordance with variations in intensity oflight impinged thereon, said light sensitive layer being light coupledto said electrolurninescent exciting layer, an electroluminescentviewing layer, a light filter layer for electrically coupling and lightcoupling said light sensitive layer to said electroluminescent viewinglayer, and a voltage source coupled across said electroluminescentviewing layer and said light sensitive layer.

3. An image display element of the type made up of a plurality ofcontiguous layers comprising the combination of an electroluminescentexciting layer, a source of video information signals coupled to excitesaid exciting layer to produce light output therefrom related to thevideo information contained in said signals, an elettroluminescentviewing layer, a layer of light sensitive material having an electricalimpedance characteristic varying substantially in accordance withvariations in intensity of light impinged thereon; said light sensitivelayer being light coupled to said exciting layer and being electricallyand light coupled to said viewing layer, the attenuation in the lightcoupling between the light sensitive layer and the viewing layer beingdiierent than the attenuation in the light coupling between the lightsensitive layer and the exciting layer, and a source of voltage coupledbetween the viewing layer and the light sensitive layer.

4. In an image display the combination comprising a plurality ofindividual signal responsive rst electroluminescent transducer means forproducing a light output related to the video information contained in asignal, a video signal source, means coupled to said video signal sourceof sweeping said plurality of transducer means with said video signal, aviewing screen comprising a plurality of second electroluminescenttransducer means for producing light output related to the amplitude ofan exciting signal, a power source, a plurality of photosensitive meanshaving light modulatable impedance characteristics, means forelectrically coupling each of said photosensitive means between saidpower source and a separate one of said second transducer means, andmeans for light coupling each of said photosensitive means to a separateone of said first signal responsive transducers.

References Cited in the le of this patent UNITED STATES PATENTS2,201,066 Toulon May 14, 1940 2,500,929 Chilowsky Mar. 21, 19502,698,915 Piper Jan. 4, 1955 2,749,480 Ruderfer June 5, 1956 2,760,119Toulon Aug. 21, 1956 2,773,992 Ullery Dec. 11, 1956 FOREIGN PATENTS157,101 Australia June 16, 1954

